1. Field of the Invention
The invention relates generally to efficient means for the generation of electrical or other power utilizing energy from geothermal sources and, more particularly, relates to arrangements including efficient super-heated steam generation and pumping equipment for long-life application in deep hot water wells for the useful transfer of thermal energy to the earth's surface.
2. Description of the Prior Art
While geothermal energy sources have been employed for the generation of power, generally known prior art systems operate with relatively low efficiency and have additional serious disadvantages. In the few installations in which substantially dry steam is supplied by wells at the earth's surface, the steam may be fed after removal of solid matter from the well head directly to a turbine. On the other hand, most geothermal wells are characterized by yields at the earth's surface of a mixture of steam and hot water along with corrosive solutes, so that the water must be separated from the steam before the latter is used in a turbine.
In both kinds of installations, relatively low pressure steam normally results, requiring employment of special turbines and yielding relatively inefficient power generation as compared to the efficiency of generation of power using normally operated fossil fuel or nuclear-powered electrical generation equipment. In only a few instances do existing geothermal wells actually produce truly super-heated steam with only minor amounts of undesired gasses and with no liquid water.
The pressure of significant amounts of liquid water in wells used with prior art geothermal systems presents other problems in addition to the separation problem. If the water is only moderately hot, extracting thermal energy from it may be expensive or, at least, inefficient. Whether or not the heat is used, the water must be handled. The water usually bears considerable concentrations of silica and of corrosive alkali salts, including chloride, sulfate, carbonate, borate, and the like ions, all of which dissolved materials present precipitation problems at the point at which any part of the water may abruptly flash into steam. If the alkaline water is allowed to escape at the installation, severe chemical and thermal pollution of streams or rivers results. Finally, there is evidence that the removal of large amounts of water from geothermal reservoirs may lead, in a generally unpredictable manner, to undesirable land subsidence in the vicinity of thermal well installations.
A major advance in the art of extraction and use of geothermal energy is reflected in the H. E. Matthews U.S. patent application Ser. No. 300,058 for a "Geothermal Energy System and Method" filed Oct. 24, 1972, issued July 23, 1974 as Pat. No. 3,824,793, and assigned to the Sperry Rand Corporation. The prior Matthews invention provides means for efficient power generation employing energy derived from geothermal sources through the generation of dry super-heated steam and the consequent operation of sub-surface equipment for pumping extremely hot well at high pressure to the earth's surface. Clean water is injected at a surface station into the deep well where thermal energy stored in hot solute-bearing deep well water is used at a deep well station to generate super-heated steam from the clean water. The resultant dry super-heated steam is used at the well bottom for operating a turbine-driven pump for pumping the hot solute-bearing well water to the station at the earth's surface. The water is pumped at all times and locations in the system at pressures which prevent flash steam formation. The highly energetic water is used at the surface station in a binary fluid system so that its thermal energy is transferred to a closed-loop surface-located vapor generator-turbine system for driving an electrical power alternator. Cooled, clean water is regenerated by the surface system and is re-injected under pressure into the well for operation of the steam turbine therein. Undesired solutes are pumped back into the earth via a separate well in the form of a concentrated brine.
In contrast to the relatively poor performance of other prior art systems, the prior Matthews invention is characterized by high efficiency as well as by other advantageous features. It is not limited to use with the rare dry steam sources, and it is devoid of the water and steam separation problems attached to prior art systems used at mixed steam and hot water supply wells. Since the novel power system operates with dry, highly superheated steam, existing efficient heat transfer elements and efficient high pressure turbines may readily be employed. According to the invention, the very large calorific content of high temperature water subjected to high pressure is efficiently employed. Since high pressure liquid is used as the thermal transfer medium, undesired flash steam formation is prevented, along with its undesired attendant deposition of dissolved materials. Because the dissolved salts are efficiently pumped back deep into the earth as remotely as need be from the geothermal source, surface pollution effects are avoided and there is relatively little risk of land sinkage in the vicinity of the geothermal source.